This invention relates to a circuit for generating touch detection signals and a locator device. More particularly, the invention relates to a locator device of such a type that a pair of electrodes in a grid pattern are scanned as a pair of capacitors whereby a touch detection signal having two peaks, one being greater and the other smaller than a specified reference level, is generated as a detection signal for the area around the touched electrode and its position is detected on the basis of this detection signal, characterized in that erroneous touch detection can be prevented by reducing or suppressing the variations in the offset of an electric current from the current outputting circuit which generates touch detection signals. The invention also relates to a circuit and a method for generating such touch detection signals.
The locator device is used as a substitute pointing device for the mouse, track ball and quick pointer on a computer system. It has a an electrostatic sensor portion comprising multiple X and Y electrodes arranged in a grid pattern and the position of a touched electrode is detected by sensing the difference in capacitance between adjacent electrodes in pair. For detecting the position of the touched electrode, X or Y electrodes are scanned, usually with a pair of adjacent electrodes taken as a set. The difference in capacitance between two capacitors formed by a pair of electrodes is detected as a difference in charge current by means of a charge current detector circuit and output as a touch detection signal.
If the X and Y electrodes in the electrostatic sensor portion are stripe electrodes thinner than the width of a finger which touches them, the capacitance of the touched electrode decreases because the electric lines of force between X or Y electrodes are interrupted by the finger. As a result, there occurs a change in the difference of capacitance between the touched electrode and the adjacent one which is either upstream or downstream of it. The difference in capacitance is positive and increases in the area upstream of the touched electrode. The difference then decreases and becomes zero in the touch position of the finger (its center portion) and thereafter increases taking a negative value. The difference then decreases to become zero again. This is the characteristic of the touch detection signal detected with the charge current detector circuit in the locator device. Briefly, the touch detection signal obtained by scanning X or Y electrode pairs with the charge current detector circuit varies in the scan direction such that two peaks occur with reference to a specified level, one being greater and the other smaller.
The touch detection signal generally described above is generated by a circuit of the type shown in FIG. 2 which receives a charge current as obtained from each electrode. FIG. 2 is a block diagram primarily for a charge current detector circuit 10 in a locator device which generates touch detection signals. Indicated by 11 is the electrostatic sensor portion (touching portion) of the charge current detector circuit; 12 is a multiplexer; 13 is a pulse drive circuit consisting of an X-side drive circuit and a Y-side drive circuit; 14 is a connection switching circuit; 15 is a differential current generator circuit; 16a, 16b and 16c are switching circuits; 17 is an integrator circuit; 18 is a control circuit; and 19 is an offset cancelling circuit. The integrator circuit 17 consists of an integrating capacitor Cs and a parallel-connected switch circuit SW for resetting the electric charges that have built up in the capacitor Cs. In the case shown, connection switching circuit 14, differential current generator circuit 15 and switching circuits 16a, 16b and 16c make up the charge current detector circuit.
Switching circuits 16a and 16b are provided between multiplexer 12 and connection switching circuit 14 and as indicated by the one-long-and-one-short dashed line, switching circuits 16a and 16b and subsequent circuits including connection switching circuit 14 are assembled in an IC. Among these circuits, switching circuit 16c is provided between differential current generator circuit 15 and integrator circuit 17.
Electrostatic sensor portion 11 is a flat member which has multiple stripe X electrodes spaced in the X direction and multiple stripe Y electrodes spaced in the Y direction; these two electrode groups are provided in a face-to-face relationship and superposed one on the other with a dielectric resin spacer interposed.
Two adjacent electrodes of either X or Y group are successively selected as a pair and driven by pulses supplied from pulse drive circuit 13. The electrodes of the other group are supplied with a voltage of constant level. The two selected electrodes of either group correspond to two capacitors Ca and Cb (see FIG. 2) in relation to the electrodes of the other group. The difference between the capacitances of these two capacitors is output as a current value from differential current generator circuit 15.
If the stripe electrodes of either X or Y group are driven by pulses from the pulse drive circuit 13, a differential pulse of positive polarity (charging current pulse) is generated in response to the rise of the drive pulse and a differential pulse of negative polarity (discharging current pulse) is generated in response to the fall of the drive pulse. Connection switching circuit 14 is used to get these two kinds of differential pulse (charge current) to have the same polarity; to this end, the connection to the input terminal at the positive phase of differential current generator circuit 15 and the connection to the input terminal at the negative phase are interchanged immediately before the rise and fall of the drive pulse. As a result, connection switching circuit 14 adjusts these two kinds of current to have a single polarity (inverts the discharging current to have a positive polarity) and outputs them to differential current generator circuit 15. The timing signal necessary for this switching operation is supplied as a timing signal T from controller 18.
When two adjacent electrodes in the Y direction as selected by multiplexer 12 are supplied with a drive pulse P, said drive pulse P is applied at one end N of each of the capacitors Ca and Cb which form a common junction (suppose N is initially for X electrodes). The other ends Na and Nb of the selected capacitors Ca and Cb (Na and Nb are initially for Y electrodes) are supplied to the (+) and (xe2x88x92) phase inputs, respectively, of differential current generator circuit 15 via multiplexer 12 and connection switching circuit 14. Differential current generator circuit 15 is composed of a Gm amplifier (transconductance amplifier) and provided at the (+) phase terminal (positive phase input terminal) and the (xe2x88x92) phase terminal (inverse phase input terminal) with the voltage signals (for charge current) that were generated at the other ends Na and Nb, respectively, of capacitors Ca and Cb. The circuit 15 outputs a differential current value representing the potential difference between the two input signals. For details of the technology about the charge current detector circuit, see U.S. Pat. No. 6,075,520 issued to the same assignee.
Offset cancelling circuit 19 is operated under the control of control circuit 18. In the absence of any signal input to differential current generator circuit 15 before scanning of electrostatic sensor portion 11 starts, offset cancelling circuit 19 sets the output of differential current generator circuit 15 at a reference level and adjusts the resulting output current to the value xe2x80x9czeroxe2x80x9d. Since the output of differential current generator circuit 15 is set to the reference level as a result of this offset cancelling operation, touch detection signals which vary along the scan direction in such a way that two peaks occur with reference to a specified level, one being greater and the other being smaller, can be obtained with high precision. Another reason for providing offset cancelling circuit 19 is to absorb variations in the reference level that occur between individual products.
To cancel the offset that may occur when no signal is input to differential current generator circuit 15, offset cancelling circuit 19 in the illustrated case is assumed to set the output voltage to Vcc/2 (Vcc is the supply voltage) and adjust the output current to zero. For offset cancelling, control circuit 18 turns switching circuit 16c on. Receiving the output signal from differential current generator circuit 15, offset cancelling circuit 19 adjusts the operating current and the like so that the output voltage is equal to Vcc/2 while the output current is zero. Switching circuits 16a and 16b remain in the initial off state, so that the input side of differential current generator circuit 15 is not connected to electrostatic sensor portion 11 and supplied with no signal.
Speaking of the current outputting Gm amplifier, it is a push-pull circuit having a current source serving as a current discharger provided upstream in the push circuit and another current source serving as a current sink provided downstream in the pull circuit. Therefore, as shown in FIG. 2, differential current generator circuit 15 has two variable current sources 15a and 15b, the first serving as the upstream current discharger and the second as the downstream current sink, and offset cancelling is performed by adjusting the current values of the two variable current sources.
After the offset cancelling, controller 18 turns switching circuits 16a and 16b on and under its control, X or Y electrodes are scanned, whereby differential current generator circuit 15 generates touch detection signals that vary with the scan direction.
A problem with the circuit configuration described above is that when electrostatic sensor portion 11 is scanned with multiplexer 12, connection to sensor portion 11 must be established by actuating control circuit 18 to turn switching circuits 16a and 16b on and the charge currents obtained by scanning have such values that they will flow toward the ground GND.
On the other hand, the capacitance between the ground and the input of differential current generator circuit 15 varies if it is connected to electrostatic sensor portion 11 which is outside the IC; as a result, the offset cancelling operation performed prior to scanning is affected to prevent complete offset cancelling. This is because the stray capacitance relative to the ground is added to the input of differential current generator circuit 15 upon connection of electrostatic sensor portion 11. Before connection of sensor portion 11, the capacitance between the ground and the input of differential current generator circuit 15 is no more than 1 pF but upon connection of sensor portion 11, it increases to as high as 30-50 pF.
This affects the offset cancelling already performed by offset cancelling circuit 19 and the setting of the reference level is no longer effective in generating precise touch detection signals. In addition, an offset in differential current generator circuit 15 shifts the reference level either upward or downward, thereby narrowing the dynamic range of detection signals having an upward peak and a downward peak that are produced from differential current generator circuit 15. As a result, it becomes difficult to determine whether a certain electrode has been touched by a finger and there occurs either erroneous detection of an untouched electrode or failure to detect a touched electrode. What is more, the position of the touched electrode cannot be detected correctly.
The present invention has been accomplished under these circumstances and has as an object providing a circuit for generating touch detection signals that can prevent erroneous detection of an untouched electrode or failure to detect a touched electrode by reducing or suppressing the variations in the current offset that occurs to a current outputting circuit which generates touch detection signals.
Another object of the invention is to provide a locator device that can prevent erroneous detection of an untouched electrode or failure to detect a touched electrode by reducing or suppressing the variations in the current offset that occurs to a current outputting circuit which generates touch detection signals.
Yet another object of the invention is to provide a method for generating touch detection signals that can prevent erroneous detection of an untouched electrode or failure to detect a touched electrode by reducing or suppressing the variations in the current offset that occurs to a current outputting circuit which generates touch detection signals.
The first object of the invention can be attained by a touch detection signal generating circuit which scans electrodes arranged in specified directions in an electrostatic sensor portion and receives a charge current obtained from each electrode to generate a detection signal that represents the touching of a specified electrode, said circuit further including an amplifier that is connected to the electrostatic sensor portion via a first switching circuit and which receives the charge current at the input terminal to generate the touch detection signal as an output current, an offset cancelling circuit which cancels an offset in the output of the amplifier by setting the output terminal of the amplifier at a specified reference level and adjusting the output current to substantially zero when the amplifier is supplied with no signal and a capacitance adding circuit which is provided between the first switching circuit and the input terminal of the amplifier and by which a capacitance equivalent to the input capacitance of input terminal of the amplifier for the case where it is connected to the electrostatic sensor portion by means of the first switching circuit is added to the input terminal of the amplifier via a second switching circuit, the offset cancelling circuit cancelling an offset in the amplifier output with the electrostatic sensor portion being disconnected from the amplifier by means of the first switching circuit and with the capacitance being added to the input terminal of the amplifier by means of the second switching circuit.
The third object of the invention can be attained by a method for generating touch detection signals, wherein after said electrostatic sensor portion is disconnected from said input terminal of said amplifier, a capacitance equivalent to the input capacitance of said amplifier for the case where said electrostatic sensor portion is connected to said input terminal is added to said input terminal, whereby said offset cancelling circuit cancels an offset in the output of said amplifier and thereafter said added capacitance is isolated from said input terminal and said electrostatic sensor portion is connected to said input terminal of said amplifier and thereafter said scanning of electrodes is performed to obtain said detection signal.
Thus, according to the invention, the input capacitance of the current outputting amplifier for the case where it is connected to the electrostatic sensor portion is preliminarily allowed for and after a capacitance equivalent to said input capacitance is added, the offset cancelling circuit is activated to cancel an offset in the output of the current outputting amplifier. Even if the added capacitance is subsequently isolated from said input terminal of the current outputting amplifier and if the electrostatic sensor portion having a similar capacitance relative to the ground is later connected, the offset cancelling realized by the offset cancelling circuit will be affected by only a suppressed degree. If electrodes are scanned under these conditions, precise touch detection signals are obtained at the output of the current outputting amplifier.
As a further advantage, the variations in the reference level for the touch detection signals that are obtained from the current outputting amplifier as electrodes are scanned can be sufficiently reduced or suppressed to expand their dynamic range.
As a result, the locator device according to the second aspect of the invention which uses the touch detection signal generating circuit provides greater ease in determining whether a specified electrode has been touched or not and this not only reduces erroneous detection of an untouched electrode and failure to detect a touched electrode but also allows for precise detection of the positional coordinates of the touched electrode.